Interferometry is an optical technique for measuring a phase shift due to variations in optical path. Typically a probe beam is split into to parts that are sent through two different paths, one part, called the probe beam travels through a path that includes a sample of interest. The other part, sometimes called a reference beam travels through a path that does not include the sample. The two beams are recombined after the probe beam has interacted with the sample and the reference beam has not. Interference of the two beams due to different optical path lengths can be detected with a photosensitive detector.
Interferometry has many applications for measurement of relatively small changes in distance, e.g., distance changes of the order of the wavelength of light used by the interferometer. Interferometry may be implemented by reflecting the probe beam off the sample. Such reflection may be done at normal incidence or at grazing incidence. Normal incidence interferometry requires a high reflectance surface. This may be impractical for certain types of samples. Grazing incidence interferometry, by contrast, can be done on a very rough surface, e.g., as rough as a business card. Grazing incidence interferometry may therefore be utilized with a greater variety of samples than normal incidence interferometry.
One example of normal incidence interferometry for measurement of the surface height and thickness variation on both sides of a wafer rapidly and accurately is described in U.S. Pat. No. 6,847,458 B2, January 2005, and also in SPIE, Advanced Characterization Techniques for Optics, Semiconductors, and Nanotechnologies III, No. 6672-1, San Diego, USA, August 2007, both of which are incorporated herein by reference in their entirety. The technique described therein combines two phase-shifting Fizeau interferometers to simultaneously obtain two single-sided distance map between each side of a wafer and corresponding reference flats, and compute thickness variation and shape of the wafer from these data and calibrated distance map between two reference flats. This technique does not place the reference plate(s) very close to the wafer so that the system is sensitive to wafer vibration and does not allow the use of light sources with short temporal coherence length.
Unfortunately, grazing incidence interferometry requires a long beam path. Light sources used in grazing incidence interferometry typically have a coherence length of a few millimeters, e.g., about three millimeters. Consequently, grazing incidence interferometry is sensitive to vibration and thermal variation in the beam path. Thermal variation can be addressed by various temperature stabilizing techniques. Vibration is particularly difficult to address at low frequencies, e.g., 100 Hz or less. In addition, the apparatus described in U.S. Pat. No. 6,847,458 requires optical components such as reference flats and collimators to be larger than the wafer in diameter. As a result of this requirement, the system is expensive, particularly for large diameter wafers (e.g., 450 millimeter diameter wafers).
Another interferometry technique described in U.S. Pat. No. 7,009,696 B2, Mar. 7, 2006, and U.S. Pat. No. 7,057,741 B2, Mar. 7, 2006, both of which are incorporated herein by reference in their entirety, is able to measure the surface height on both sides and thickness variation of a wafer. This technique combines two grazing incidence interferometers, simultaneously obtaining front- and backside topography data, and computes thickness variation and shape of the wafer from these data. Multiple measurements of portions of the wafer are stitched together to obtain full wafer topography data maps. A flat bar in close proximity to portions of one side of the wafer provides a damping arrangement reducing unwanted wafer vibrations during measurement.
Unfortunately, this technique has a long, non-common optical path length between the object being measured and the reference which makes it susceptible to air temperature gradients (and resulting air turbulence). The damping arrangement does not cover the entire surface area of the wafer and is applied only on one side of the wafer. Consequently, the damping is generally regarded as less effective. Furthermore, this technique uses a grazing incidence optical arrangement, which results in relatively low accuracy and precision measurements. In addition, system to system matching is difficult with such an optical arrangement.
It is within this context that embodiments of the present invention arise.